function ListNode (val) {
    this.val = val ;
    this.next = null;
}
const n1 = new ListNode(1) ;
const n2 = new ListNode(2) ;
n1.next = n2;
const n3 = new ListNode(3) ;
// n1后面插入
//n3.next = n1.next ;
//n1.next = n3 ;  
// n2前面插入
const target = n1.next;
n1.next = target.next;
